Monolithic cathode structure



T 7 W E A W a a N 4 a 7 H v 5 7m 6 m M P. CHORNEY ET AL MONOLITHIC CATHODE STRUCTURE Filed June 17, 1964 K ff? f.

5 Z k 7 in MM w m W WM v m (J Mk 5 L 00 wm 0 7 T6 0 ML 3 v 44 4. U A; 5 Z A M 5 0 Z 25 L 7 W 55. m 5 WK U E am m 3 [KW Z N5 Feb. 7, 1967 United States Patent O 3,303,378 MONOLITHIC CATHODE STRUCTURE Paul (Ihorney, Norwood, and John Valun, Framingham,

Mass, assignors, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Filed June 17, 1964, Ser. No. 375,986 1 Claim. (Cl. 313-346) This invention relates to a cathode construction and composition for resisting sputtering more effectively than cathodes known and used heretofore and more particularly to improvements in cathodes for use in low-pressure, high density plasma devices.

In hot cathode discharge tubes, sputtering of the cathode is a severe problem. Sputtering caused by ion bombard'ment of the cathode eventually poisons the cathode, renders the tube inoperative by depositing sputtered metal on insulating surfaces, and alters mechanical size and dimensional accuracy of the cathode assembly. In lowpressure devices empolying high density plasmas, e.g., in beam plasma amplifiers, sputtering and rapid cathode destruction have been severe problems.

A standard cathode form comprising a thin nickel base coated with a standard mixture of barium and strontium oxides deteriorates in a short time under ion bombardment in plasma applications. In a short time, the oxide coating darkens, cracks, and flakes off leaving the surface of the nickel base essentially bare of oxide. This results in erratic operation of the plasma, drift of operation, and heavy coating of metallic film onto the glass walls of the tube. Cathodes, oxide-coated as well as dispenser-type, in the form of thin metallic disks swaged into or welded to an end of a thin-walled metal cylinder support holder have proven to be unsuitable for low-pressure high-density plasma discharge devices. The metal cylinder support holder in combination with the emitting disk defines a cup-shaped cathode structure to serve as a housing for the heater filament.

An object of the invention is to reduce sputtering in lowpressure, plasma discharge devices.

A further object is to provide an electrode structure subject to substantially less sputter and less deterioration by ion bombardment than structures known in the art heretofore.

A further object is to provide a superior, rugged, more practical, more stable, more reliable, more durable, comparatively inexpensive, and easy to fabricate cathode.

Other objects and advantages will appear from the following description of an example of the invention, and the novel features will be particularly pointed out in the appended claim.

FIG. l is a cross-section of a plasma-type discharge device which can embody this invention, and

FIG. 2 is an enlarged cross-sectional view of a cathode for the device shown in FIG. 1, embodying the teachings of this invention.

In FIG. 1, there is shown a low-pressure high-density plasma discharge device of the Penning-type, described in US. Patent 2,182,736. The device 10 includes a glass envelope 12 having a main circular cylindrical portion 14 with an arbitrarily-located tubulation 20 extending from the main portion of the envelope. The end of tubulation 2% is joined to a gas reservoir, not shown. An electrode assembly 22 is secured in each end of the cylindrical portion 14 of the envelope. Each electrode assembly includes a flat annular anode 24 bonded to and mechanically supported within envelope portion 14 by rigid leads 26 and 26a, a cup-shaped dispenser-type cathode 28 bonded to and mechanically supported coaxially with anode 24 by rigid lead 30 which join lead 30a, and a coil of heater wire 3,303,378 Patented Feb. 7, 1967 lCC 32 mechanically supported within the cathode cup and electrically connected to the interior of the. cathode cup and bonded at the other end to lead 34 which joins lead 34a. A magnetic field generating means 42 is shown in outline surrounding the envelope for generating the longitudinal field indicated by the vector B.

The apparatus shown in FIG. 1 is utilized in plasma studies. Cathodes made in accordance with the state of the art and used in a device of the type shown in FIG. 1 deteriorate rapidly, accompanied by deposition of sputtered cathode material on the glass walls of the tube making it diflicult to obtain reliably reproducible data.

Sputtering occurs because the cathode structure undergoes severe ion bombardment and the sputtered material deposits on the envelope. This is particularly pronounced in low-pressure, high-density plasma discharge devices because of longer mean-free paths of positive ions. Ion bombardment cannot be eliminated because ions are an integral part of a plasma. Prolonged sputtering eventually results in undesirable mechanical and electrical effects,

' e.g. electrical leakages along insulating surfaces, obstruction of viewing, dimensional changes, etc.

In accordance .with this invention, the cathode 28, shown on a larger scale in FIG. 2, is fabricated as a monolithic structure of a dispensing type cermet, i.e., a sintered compact of a mixture of a metal powder and a non-metal emitter. One example of a suitable material is a nickel cermet which includes nickel and barium oxide. The nickel cermet is produced by hydraulically compacting a mixture of powdered nickel and alkali earth carbonates into slugs which are later sintered. ,The resulting material is conveniently machinable and has proven to be rugged and reliable as an emitter. One mixture that has given satisfactory results consists of nickel and 10% commercially available mixture of alkali-earth carbonates, commonly know as radio mix. It is postulated that other proportions of the constituents of the cermet would be satisfactory. Sintered tungsten impregnated with barium and calcium aluminate is another example of a suitable cermet material for cathodes of the :type encompassed by this invention.

The cup-shaped cathode shown in FIG. 2 is formed by machining a cermet slug. If a cathode disk only is fabricated of the cermet and the cylindrical support holder of the cathode is made of metal, and the disk is swaged or welded into one end of the holder, and the combination is operated as a cathode subject to ion bombardment, though no gross mechanical deterioration of the cermet disk occurs under the ion bombardment, sputtering of metal is just as heavy as in prior art cathode structures. With the cathode formed as a monolithic cermet structure, sputtering is reduced many fold under the same conditions of ion bombardment. Apparently the supporting cylinder in prior art cathode assemblies was responsible for most of the sputtering; the cermet material is highly resistant to sputtering and poisoning. Even under prolonged ion bombardment, cathodes formed as described are stable and long-lived.

A possible explanation for the fact that the surface of nickel cermet, for example, has low sputter is that ions bombarding the surface do not encounter just the metal of the matrix but an admixture of metal and non-metal. The non-metal having a higher latent heat of evaporation is more difficult to sputter and thereby forms a protective coating around the more sputter-susceptible nickel of the matrix.

In other words, the surface is effectively a mosaic of metal and non-metal; hence less metallic area is exposed per unit area to the direct ion bombardment. Also, the density of metal in the matrix is much less than in pure metal since the metal in the matrix is a porous material. The presence of the pores reduces the sputter yield. Energetic ions may enter pores and sputter metal but the sputtered metal mainly deposits on the walls of the pores.

This invention may also be embodied in cold cathode discharge devices; it has utility WhGIfiVfil' low sputtering is desired from an electrode surface subject to ion bombardment, e.g. as in ion-propulsion applications.

It is surmised that a metal matrix impregnated with any oxide or other compound with a high latent heat of evaporation forms a sputter resistant surface though not necessarily with good emission properties. Since surfaces other than emitting or cathode surfaces are subjected to ion bombardment, such surfaces may be protected with an impregnated material of high latent heat of evaporation.

It will be understood that various changes in the details, materials and arrangements of parts (and steps), which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claim.

We claim:

A hot-cathode, low-sputter, high-density plasma type discharge device comprising:

an elongated envelope,

a tubulation extending from the envelope for connection to a reservoir of low pressure ionizable gas,

a monolithic cup-shaped cathode made of a sintered compact mixture of nickel powder and a powder of a thermionic emissive material,

.a rigid conductor lead secured to the outer surface of the monolithic cup-shaped cathode and extending longitudinally from the cathode through an end of the envelope to support the cathode with its outside end surface directed toward the other end of the envelope,

a heater coil within said cup-shaped cathode, one end of said heater coil being in physical engagement and electrical contact with the cathode at an inner end portion thereof, 7

a rigid conductor lead secured to the other end of the heater coil, supporting the heater coil and extending therefrom through said end of said envelope, and

an anode supported within said envelope in line with said cathode.

References Cited by the Examiner UNITED STATES PATENTS 2,182,736 12/1939 Penning 313-162 X 2,700,118 1/1955 Hughes et a1 313-346 2,895,070 7/1959 Espersen 313346 2,975,320 3/1961 Knauer 313346 X 3,013,171 12/1961 Beck 313--346 X 3,076,916 2/1963 Koppius 313-346 3,134,924 5/1964 Henderson et a1. 313--346 3,155,864 -1l/1964 Coppola 313346 JOHN W. HUCKERT, Primary Examiner.

A. JAMES, Assistant Examiner. 

